Recirculation of exhaust gases has been developed as a method for inhibiting formation of oxides of nitrogen during the combustion process in an internal combustion engine. In general, it is desired to recirculate exhaust gases at a rate proportional to the rate of induction air flow. To accomplish that purpose, exhaust gas recirculation (EGR) control assemblies have included an EGR control valve pintle positioned to provide exhaust gas recirculation at rates which maintain the pressure in the EGR passage upstream of the pintle equal to a reference pressure. Recirculation of exhaust gases has thus been varied with exhaust backpressure, which in turn varies as a function of induction air flow, to provide exhaust gas recirculation substantially proportional to induction air flow.
Such prior EGR control assemblies generally included a transducer for regulating a subsatmospheric operating pressure by which the control valve pintle was positioned. The transducer employed an air bleed valve to regulate the operating pressure--opening an air bleed to increase the operating pressure which caused the control valve pintle to reduce exhaust gas recirculation when the induction air flow (and thus the engine exhaust backpressure) decreased and the control pressure accordingly started to fall below the reference pressure, and closing the air bleed which reduced the operating pressure and caused the control valve pintle to increase exhaust gas recirculation when the induction air flow (and thus the engine exhaust backpressure) increased and the control pressure accordingly started to rise above the reference pressure. The bleed valve was carried on a control diaphragm subjected on one side to the control pressure in the EGR passage and balanced by atmospheric pressure on the opposite side and by the bias of a spring; the combination of atmospheric pressure and the spring bias formed the reference pressure.
Various controls have been adopted to cancel the operating pressure used by such assemblies and thus entirely preclude exhaust gas recirculation under conditions such as idle, wide open throttle and low temperature operation. For other selected conditions such as heavy load operation, however, it may be desired to provide exhaust gas recirculation in relatively high proportion to induction air flow, while for conditions such as light load operation it may be desired to provide exhaust gas recirculation in relatively low proportion to induction air flow.
One proposal for changing the proportion of exhaust gases recirculated involved varying the reference pressure with engine operating conditions--increasing the reference pressure to effect an increase in the control pressure and thus reduce exhaust gas recirculation when a lower proportion is desired, and decreasing the reference pressure to effect a decrease in the control pressure and thus increase exhaust gas recirculation when a higher proportion is desired. That proposal is set forth in copending application Ser. No. 879,781 filed Feb. 21, 1978 in the name R. J. Haka.
Another proposal for changing the proportion of exhaust gases recirculated involved maintaining the pressure in a restricted pressure zone of the recirculation passage equal to the reference pressure when a low proportion is desired and maintaining the pressure in an unrestricted pressure zone of the recirculation passage equal to the reference pressure when a high proportion is desired--the proportion then being determined by the size of the pressure zone. That proposal is set forth in copending application Ser. No. 929,653 filed July 31, 1978 in the name of D. D. Stoltman.